A resistor is a small part in an electric circuit whose job is to resist the flow of current — to make it harder for electricity to pass, so the current falls to a safe or useful level. It is often a little cylinder with coloured rings, or a tiny block on a board. Physics class and repair shops use the word; it is not a metaphor in ordinary talk. You do not call a stubborn person a resistor. Though actually, people sometimes say resistor when they mean resistance (the property, measured in ohms) or even transistor (a different part, used for switching and amplifying). The -or here names the thing that does the resisting.